Actuators are parts that convert stored energy into movement, and in that way are like the “muscles” of a robot. Current conventional robots use high stiffness actuators, or powered joints, to provide absolute positioning accuracy in free space. For example, in traditional manufacturing operations where robots perform tedious and repetitious tasks in a controlled environment with great speed and precision, position controlled robots that stiffly follow predefined joint trajectories are optimal. Traditional position controlled actuators are designed from the premise that stiffer is better. This approach gives a high bandwidth system, but is prone to problems of contact instability, noise, and low power density.
Variable stiffness actuators provide many benefits in force control of robots in constrained, unstructured environments. In unstructured environments, where little is known of the environment, force controlled joints or variable stiffness actuators are desirable because they allow a robot to comply with its surroundings. Such robots can execute dynamic activity in a changing and unpredictable environment—e.g., humanoid robots, legged robots walking over rough terrain, robotic arms interacting with people, wearable performance-enhancing exoskeletons, haptic interfaces, and other robotic applications.
Variable stiffness actuators provide benefits including shock tolerance, lower reflected inertia, more accurate and stable force control, extremely low impedance, low friction, less damage to the environment, and energy storage. However, current variable stiffness actuators available in the art do not provide an adequate range of stiffness required for many applications. For example, currently-available actuators are only capable of obtaining a ratio of highest stiffness to lowest stiffness in the range of about 10. Moreover, many current variable stiffness actuators cannot provide adequate maximum stiffness, especially for a full range of motion. Furthermore, many current variable stiffness actuators are too slow in adjusting their stiffness to adequately perform their function.